The use of radiofrequency (RF) generators with electrodes applied near or in neural tissue for pain relief or functional modification is well known. For instance, the RFG-3C RF Lesion Generator available from Radionics, Inc., Burlington, Mass. can be used with electrodes placed near neural tissue to heat the tissue by RF resistive power dissipation of the generator power in the tissue. Thermal monitoring by a thermo sensor in the electrode is used to control the process. Heat lesions produced at tissue temperatures of 60 to 95 degrees Celsius (.degree. C.) are common. Tissue dies by heating at about 45 to 50.degree. C., so heat lesion generation is designed to elevate the neural tissue above this lethal temperature threshold. Often, the procedure of heating above 45 to 50.degree. C. causes severe pain to the patient, which is so unpleasant (and frequently unbearable) that a local or general anesthetic is required during the heat procedure. Use of anesthesia poses a degree of undesired risk to the patient, and the destructive nature of and unpleasant side effects of the RF heat lesion are limitations of this technique.
Heat lesion generators typically use continuous wave RF generators with radiofrequencies of between 100 KiloHertz to several MegaHertz (viz. the RF generators of Radionics, Fischer, OWL, Elekta, Medtronic, Osypka, EPT companies). The theory and use of RF lesion generators and electrodes for pain and functional disorders is described in various papers (see, e.g., (1) Cosman, et al. "Theoretical Aspects of Radiofrequency Lesions and the Dorsal Root Entry Zone," Neurosurg 15:945-950, 1984; and (2) Cosman E R and Cosman B J. "Methods of Making Nervous System Lesions," in Wilkins R H, Rengachary S S (eds): Neurosurgeny. New York, McGraw-Hill, Vol. III, 2490-2498, 1984).
Neural stimulation is also now a common method of pain therapy. Stimulus generators with outputs of 0 to 10 volts (or zero to several milliamperes of current are used) are typical. A variety of waveforms and pulse trains in the "physiologic" frequency ranges of 0 to about 300 Hertz is also typical. This output is delivered to electrodes placed near or in neural tissue on a temporary basis (acute electrode placement) or permanent basis (chronic electrode implants). Such stimulation can relieve pain, modify neural function, and treat movement disorders. Typically, the stimulation is sustained to have a long-term effect, i.e., usually when the stimulus is turned off, pain will return or the therapeutic neural modification will cease after a short time (hours or days). Thus permanent implant electrodes and stimulators (battery or induction driven) is standard practice (e.g., the commercial systems by Medtronic, Inc., Minneapolis, Minnesota), and the stimulus is usually sustained or repeated on an essentially continuous basis for years to suppress pain or to treat movement disorders (viz. Parkinsonism, bladder control, spasticity, etc.). Stimulators deliver regular pulse trains or repetitive bursts of pulses in the range of 0 to 200 Hertz (a physiologic range similar to the body's neural frequency pulse rates), so this method simulates or inhibits neural function at relatively low frequency. It does not seek to heat the neural tissue for destructive purposes as in the high frequency technique. Chronically or permanently implanted stimulators often require battery changes, long-term maintenance and patient follow-up, which are expensive and inconvenient, often requiring repeated surgery.
Electrosurgical generators have been commonly used for decades for cutting and coagulating tissue in surgery. They typically have a high frequency, high power generator connected to an electrode that delivers a high power output to explode tissue for tissue cutting and to cook, sear, and coagulate tissue to stop bleeding. Examples of such generators are the generators of Codman, Inc., Randolph Mass., Valley Labs, Inc., Boulder, Colo., and EMC Industries, Montrouge, France. Such generators have high frequency output waveforms that are either continuous waves or interrupted or modulated waves with power controls and duty cycles at high levels so that tissue at the electrode is shattered and macroscopically separated (in cutting mode) or heated to very high temperatures, often above cell boiling (100.degree. C) and charring levels (in coagulation or cauterizing mode). The purpose of electrosurgery generators is surgical, not therapeutic, and accordingly their output controls, power range, duty cycle, waveforms, and monitoring is not designed for gentle, therapeutic, neuro-modulating, sub-lethal temperature application. Use of an electrosurgical unit requires local or general anesthesia because of its violent and high-temperature effect on tissues.